For many species of microorganisms, invasion and survival within mammalian cells is central to establishing a successful host-parasite relationship. This localization within host cells may protect the microorganism from host defenses and permit the microorganism to cross epithelial barriers and subsequently become systemically distributed. The precise mechanisms by which bacteria enter host tissues have been unclear. The invasive character of pathogens, while deleterious to the health and viability of host cells, does provide a mechanism for transfer of molecules and aggregates across an intact cellular membrane. Thus, if the invasive quality could be transferred to an innocuous microorganism strain, such a strain could serve as a vehicle for transporting molecules of interest into the cytoplasm and organelles of host cells.
There is a further consideration in that the microorganism could provide for the transfer of genetic material into the mammalian host cell. In this manner, novel genetic capabilities could be imparted to the host cell. One capability of interest would be the synthesis of surface membrane proteins or envelope proteins of pathogens. These proteins would then serve as antigens to provide a strong immune response, without the host having to suffer the effects of infection by the pathogen.